1. Field of the Invention
The present invention relates to a toner used for the development of a latent image formed in electrophotography, electrostatic recording method, electrostatic printing method or the like.
2. Discussion of the Related Art
Conventionally, carbon blacks have been widely used as a black colorant for a toner. However, the following problems have been addressed. Specifically, the toner containing carbon black has a low resistance, so that an appropriate level of triboelectric charge properties cannot be obtained, whereby deteriorations in image qualities such as unevenness of solid images and toner scattering are likely to be generated. Especially in a high-speed machine for a two-component development having a linear speed exceeding 370 mm/sec, a centrifugal force applied to the toner by a magnet roller is greater than the Coulomb's force between the toner and the carrier, so that there arises a phenomenon in which a toner is dropped in lumps to a so-called non-image bearing portion (hereinafter referred to as a “toner drop”). Especially in a high-printing mode in which printed ratio exceeds 20%, the toner consumption is rapid, so that the toner cannot have a sufficient level of triboelectric charges, whereby the toner drop becomes more remarkable.
In addition, in a monocomponent development, since the frictional triboelectric charging time is very short, the toner containing carbon black cannot have a sufficient level of triboelectric charges. Therefore, there arises undesired phenomena such that the fixed images are blurred during solid-image printing, and that the background fogging is likely to take place.
Various raw materials which are used in place of carbon black as black colorants have been studied. For instance, Japanese Patent Laid-Open No. Hei 4-356059 (U.S. Pat. No. 5,262,264) and Hei 5-19536 disclose a black toner comprising a yellow pigment, a mazenta pigment and a cyan pigment, without containing carbon black. In addition, Japanese Patent Laid-Open No. 2000-10344 (U.S. Pat. No. 6,130,017) and Japanese Patent Laid-Open No. Hei 9-25126 disclose toners comprising various composite oxides. However, even when the three pigments are mixed together, an appropriate degree of blackness cannot be obtained, and the triboelectric charges are also unstable. In addition, although the inclusion of the latter composite oxide contained in a toner gives the toner a high triboelectric charge, it is deficient in the stability of the triboelectric charges. Also, since the degree of blackness is also slightly lower, there arises a problem of lowering the fixing ability when the composite oxide is contained in a large amount in a toner.
An object of the present invention is to provide a toner having excellent stability and initial rise in the triboelectric charges, thereby continuously giving an excellent fixed image without generating deterioration in the image quality even when the toner is applied to a high-speed machine for a two-component development or to a developing device for a nonmagnetic monocomponent development.
These and other objects of the present invention will be apparent from the following description.